OR

If the sales value £1000 or less then the commission is 0%

From £1001 to £2999 the commission is 20%

From £3000 to £5999 the commission is 25%

Above £6000 the commission is 33%

Do you just apply the commission as a straight value or in blocks

Assuming the value in A1

=IF(A1<=1000,0,IF(A1<3000,(A1-1000)*0.2,IF(A1<6000,(A1-3000)*0.25+(1999*0.2),IF(A1>=6000,(A1-6000)*0.33+(2999*0.25)+(1999*0.2),"incorrect Value"))))

I have used a nested IF

If the sales value £1000 or less then the commission is 0%

IF(A1<=1000,0,

From £1001 to £2999 the commission is 20%

IF(A1<3000,(A1-1000)*0.2,

As its less than 3000 , then we need to take 1000 away from whatever is in A1 and that gets 20%

IF(A1<6000,(A1-3000)*0.25+(1999*0.2),

Less than 6000

We get 1999 at 20%

and what ever is above 3000 at 25%

IF(A1>=6000,(A1-6000)*0.33+(2999*0.25)+(1999*0.2),

Greater or equal to 6000

we get 1999 at 20%

we get 2999 at 25%

and what ever is over 6000 at 33%

You may want to review the thresholds, and using the same formula you can change if you meant to use different numbers